Coating medium

ABSTRACT

A heat curable coating medium for use in the formation of a cured, highly cross-linked, polymeric coating on a substrate. The coating medium comprises an aromatic diamide diacid diamine formed as the reaction product of an aromatic diamine and an aromatic dianhydride, in the molar ratio of two-to-one respectively, in a solvent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No. 475,483filed June 3, 1974, now U.S. Pat. No. 4,003,947 which is acontinuation-in-part of copending application Ser. No. 467,615 filed May6, 1974 which is, in turn, a continuation of application Ser. No.822,899 filed May 8, 1969, now abandoned, and the contents of suchcopending and abandoned applications are specifically incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to coating compositions for producingpolymeric coatings on substrates, and more particularly to aqueous basedpolymeric coating compositions produced from the reaction product ofaromatic diamines and aromatic dianhydrides. More specifically, theinvention relates to a coating composition, a process for producingcoating compositions of the foregoing character, a process for coatingsubstrates therewith, coatings produced thereby, and to coatedsubstrates.

BACKGROUND OF THE INVENTION

Polyamide and polyimide coating materials and coatings producedtherefrom are generally well known in the art. See, for example, U.S.Pat. No. 3,652,500, issued Mar. 28, 1972, to M. A. Peterson, for"Process For Producing Polyamide Coating Materials By End Capping"; U.S.Pat. No. 3,663,510, issued May 16, 1972, to M. A. Peterson, for "ProcessFor Producing Polyamide Coating Materials"; U.S. Pat. No. 3,507,765,issued Apr. 21, 1970, to F. F. Holub and M. A. Peterson, for "Method ForElectrocoating A Polyamide Acid"; U.S. Pat. No. 3,179,614, issued Apr.20, 1965, to W. M. Edwards, for "Polyamide Acids, Compositions Thereof,And Process For Their Preparation"; U.S. Pat. No. 3,179,634, issued Apr.20, 1965, to W. M. Edwards, for "Aromatic Polyimides And The Process ForPreparing Them"; and U.S. Pat. No. 3,190,856, issued June 22, 1965 to E.Lavin, et al, for "Polyamides From Benzophenonetetracarboxylic Acids AndA Primary Diamine". The prior art involves generally the preparation ofa coating medium containing a high molecular weight polyamide acid, andapplication of the coating medium to a substrate to provide a polyamideacid coating thereon, followed by the curing of the high molecularweight polyamide acid to a polyimide. To the extent necessary for a morecomplete understanding of the present invention, reference should bemade to the above listed patents, the disclosures of which are hereinincorporated by reference.

Commercially available coating materials for use in electricalapplications, such as the coating materials disclosed in U.S. Pat. No.2,936,296, issued May 10, 1960 to F. M. Precopio and D. W. Fox for"Polyesters From Terephthalic Acid, Ethylene Glycol and a HigherPolyfunctional Alcohol", and used and sold commercially under thetrademark "ALKANEX" by General Electric Company, are widely used, highlysuccessful and effective compositions, but are believed to have oneeconomic disadvantage in that they involve and require the use oforganic solvents. Where organic solvents are used, they are driven offduring cure and are generally not economically recoverable. It istherefore ecologically and environmentally desirable to utilizesubstantially water based solvents.

Aqueous base polyamide acid systems of the type described in theabove-mentioned patents to Peterson result in high temperatureelectrical grade coatings (230° C., 20,000 hr. class insulation coating)are stable, and easily made and used, but are believed to be quiteexpensive compared to the polyester compositions. Aqueous base acrylicsystems, of the type described in U.S. Pat. No. 2,787,603, issued Apr.2, 1957 to P. F. Sanders for "Aqueous Coating Compositions andSubstrates Coated Therewith," while inexpensive, are believed to be notgenerally suitable for high temperature electrical grade coatingsapplications such as class B applications (i.e., 130° C., 20,000 hours).Moreover, such aqueous base acrylic systems are emulsions and notsolutions, thereby creating certain stability problems.

In water base systems, because of the high latent heat of vaporization,it is believed to be desirable to utilize as high a solids content as ispossible commensurate with workable viscosities because the medium mustbe used with automatic coating apparatus such as wire towers. Highmolecular weight polymers, such as the polyamide polymers which aredescribed in the patents listed above, produce extremely viscoussolutions except in relatively low solids content systems. For manyapplications the low solids content systems are quite suitable. For highspeed wire tower use, however, the low solids content aqueous solutionis believed to create production problems which reduce the efficiency ofthe tower and in some cases result in under-cured coatings.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide acomposition for application from either an organic or an aqueous basedmedium of high solids content to form a coating on a substrate, whichcoating may, upon curing, be converted to a highly cross-linkedpolymeric coating.

A more specific object of the present invention is to provide a new andimproved coating material for producing a flexible, elastic, clear,adherent, nontacky dielectric coating on a substrate.

Another object of the present invention is to provide a new and improvedmonomeric coating medium which may be applied as a thin film coating ona substrate from either an organic solvent solution or anaqueous-organic solvent system and which when cured produces a coatinghaving characteristics of a polyimide-polyamide type coating.

A further object is to provide a coating medium based on the reactionproduct of an aromatic diamine and an aromatic dianhydride, which can beapplied from an aqueous based system, and which is curable to form aflexible, dielectric, clear, temperature-resistant, adherent coating ona substrate.

Still another object of the present invention is to produce a coatingmedium which is aqueous based, has a high solids content and which, whenapplied as a film coating to a substrate, is curable to form a flexible,clear, temperature-resistant, dielectric, adherent coating having thecharacteristics and properties of a polyimide-polyamide type coatingmaterial.

Still a further object of the invention is to provide a novel coatingmaterial which can be prepared in a high solids content organic reactionsolution, and subsequently diluted with water to a viscosity suitablefor coating without hydrolyzing the material. More particularly, it isan object of this invention to provide a material of the foregoing typewhich, when applied as a coating to a substrate from an aqueous solutionis readily and efficiently heat curable to form a highly cross-linked,flexible, clear, adherent, dielectric, temperature-resistant polymercoating on the substrate.

These as well as other objects and advantageous features of theinvention will be in part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION

In accordance with the foregoing objects, the present inventioncontemplates a coating composition, particularly with a substantiallyaqueous base, embodying a low molecular weight monomeric compoundproduced as the reaction product of an aromatic diamine and an aromaticdianhydride in the molar ratio of two-to-one respectively. The initialreaction takes place in an anhydrous water miscible solvent which isnonreactive with the diamine and dianhydride reactants.

The diamine is first dissolved in the solvent and then the dianhydrideis slowly added to form a diamide diacid diamine monomer reactionproduct in the solvent system. To provide an aqueous base system, thereaction product, in the water miscible organic solvent system, isreacted with a volatile base, such as ammonia or a primary or secondaryamine, to produce a water soluble compound. Water is then added toprovide an essentially aqueous based coating medium of the desiredsolids content. The initial reaction is carried out at below imidizationtemperatures, generally below about 70° C.

Upon application of a coating of the medium to a substrate, the coatingmay be cured at a temperature between 100°-500° C. to drive off thewater and solvent and polymerize the monomer to provide a generallyladder structured polymeric coating, which is clear, flexible,non-tacky, dielectric, and exhibits excellent adhesion to the substrate.Unexpectedly the coating film thus produced exhibits the foregoingproperties, that is properties which are generally characteristic ofpolyamide and polyimide type coatings. Moreover, it has been discoveredthat the aqueous based coating medium is stable and neither gels norcoagulates nor forms a precipitate upon standing.

The coating medium thus produced is not only highly effective for diptype coating operations, but is also effective for use in electrocoatingapplications. For electrocoating, the medium may be further diluted withwater to an appropriate consistency without affecting the monomer or itspolymerization.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The coating composition of the present invention embodies a lowmolecular weight monomeric compound produced as the reaction product ofan aromatic diamine and an aromatic dianhydride in the molar ratio ofabout two-to-one respectively. The initial reaction takes place in anaprotic solvent system which is nonreactive with or inert to the diamineand dianhydride reactants. The reaction is carried out at a temperaturebelow about 70° C. so that there is a negligible level of imidizationresulting in the ortho amide acid product, generally characterized as adiamide diacid diamine (see U.S. Pat. Nos. 3,652,500 and 3,663,510above). If the reaction solution is heated under controlled conditions,certain desired levels of imidization can be achieved. However, if theheating is carried too far, such as to a level greater than about 90%,depending upon the particular diamine and dianhydride selected, theimide thus formed precipitates as an insoluble, inflexible, unreactivesolid precipitant.

Quite unexpectedly, it has been discovered that if a thin film is formedof the monomeric reaction product prior to the application of heat andthe film is heated, a clear, tough, flexible, highly cross-linkedpolymeric film is produced. The present invention is concerned with theproduction of such a film by forming a diamide diacid diamine monomer,applying a coating of that monomer to a substrate and curing the coatingto produce the polymer film.

The coating medium may be an organic based or an aqueous based medium.To the latter end, following the formation of the reaction product ofthe diamide diacid diamine in an organic solvent system, a volatile baseis added in an amount sufficient to react with that reaction product toproduce a water soluble compound. The system is then diluted with waterto provide an essentially aqueous based coating medium. A coating of themonomeric reaction product in the aqueous medium is then applied to asubstrate as a thin film and the coating film is then cured to convertit to a highly cross-linked polymeric substance.

The initial reaction between the diamine and the dianhydride is carriedout in a high solids content organic solvent system, with the reactantsin the molar ratio of two-to-one respectively, that is in the molarratio of two moles of aromatic diamine to one mole of aromaticdianhydride. To this end, the diamine, in the proportion of two moles,is first dissolved in the organic solvent. The dianhydride, inproportion of one mole, is then slowly added or trickled into thediamine solution. The temperature is maintained generally at about 70°C. or below, and preferably at about 50° C. or below. As the dianhydrideis trickled into the diamine solution, one mole of the dianhydrideimmediately reacts with two moles of the diamine to produce the diaminediacid diamine monomeric coating material desired. It has been observedthat, if the one mole of dianhydride is dissolved first, and the twomoles of diamine is next charged, polymerization occurs resulting in ahigher molecular weight material. On the other hand, if the dianhydrideis added rapidly, such as in a chunk or as a slug, the dianhydridereacts faster than it dissolves, thereby leaving "islands" of unreacteddianhydride surrounded by reacted dianhydride.

It has further been discovered that the diamide diacid diamine reactionproduct, that is the monomer or "polymer precursor", may be converted toan aqueous based system by the addition of a volatile base in an amountsufficient to convert the reaction product to a water soluble form,followed by dilution of the system with water to form an aqueous-organiccoating medium, without hydrolyzing or destroying the diamide diaciddiamine monomer. This reaction is generally initially carried out in theorganic solvent at a solids level greater than 40% solids by weight, andmore often greater than 50% solids by weight. Upon the addition of thevolatile base and subsequent dilution with water, the solids content ofthe aqueous-organic system is reduced to a level suitable for use incoating applications. A substantial savings can thus be realized in theorganic solvent required to initially produce the monomer. This isaccomplished because of the use of a high solids content reactionsolution, and the subsequent use of water to dilute the system to asuitable coating consistency.

A coating of the monomeric diamide diacid diamine reaction product isthen applied to a substrate such as sheet, wire, etc., from either anorganic solution or, more preferably from an environmental andecological standpoint, from an aqueous solution. As pointed out above,whereas elevated temperatures result in imidization of the solution andprecipitation of the monomer as an insoluble, inflexible imide, it wasunexpectedly found that, upon the application of a coating of themonomer to the substrate as a thin film, the coating film is curable ata temperature sufficient to convert the coating to a polymeric, highlycross-linked flexible film material. Unexpectedly, the film materialthus produced is highly flexible, tough, clear, of good dielectricstrength, and suitable for electrical applications.

The aromatic dianhydrides that are useful in accordance with thisinvention are those having the generally formula: ##STR1## wherein R isa tetravalent radical containing two benzene rings joined by achemically inert, thermally stable moiety selected from the groupconsisting of an alkylene chain having from 1 to 3 carbon atoms, analkyl ester, a sulfone and oxygen, each pair of carboxyl groups beingattached to different adjacent carbon atoms of a single separate ring.These dianhydrides include, for example,

4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate,

3,3',4,4'-benzophenonetetracarboxylic dianhydride,

bis(3,4-dicarboxyphenyl) sulfone dianhydride,

bis(2,3-dicarboxyphenyl) methane dianhydride,

2,2-bis (3,4-dicarboxyphenyl)propane dianhydride,

bis (3,4-dicarboxyphenyl)ether dianhydride,

2,2-bis(2,3-dicarboxyphenyl)propane dianhydride,

1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,

1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride,

and the like.

The aromatic diamines that are useful in accordance with this inventionare those having the general formula:

    H.sub.2 N-R'--NH.sub.2

wherein R' is a divalent radical selected from the group consisting of##STR2## wherein R'" and R"" are an alkyl or an aryl group having 1 to 6carbon atoms, n is an integer of from 1 to 4 and m has a value of 0, 1or more and ##STR3## wherein R" is selected from the group consisting ofan alkylene chain having 1-3 carbon atoms, ##STR4## wherein R'" and R""are as above-defined and x is an integer of at least 0. In general, thediamines contain between 6 and 16 carbon atoms, in the form of one ortwo six membered rings.

Specific diamines which are suitable for use in the present inventionare:

m-phenylene diamine,

p-phenylene diamine,

4,4'-diaminodiphenyl propane,

4,4'-diaminodiphenyl methane,

benzidine,

4,4'-diaminodiphenyl sulfide,

4,4'-diaminodiphenyl sulfone,

3,3'-diaminodiphenyl sulfone,

4,4'-diaminodiphenyl ether,

2,6-diaminopyridine,

bis-(4-aminophenyl)diethyl silane,

bis-(4-aminophenyl)phosphine oxide,

bis-(4-aminophenyl)-N-methylamine,

1,5-diamino naphthalene,

3,3'-dimethyl-4,4'-diamino-biphenyl,

3,3'-dimethoxy benzidine,

m-xylylene diamine,

1,3-bis-delta-aminobutyltetramethyl disiloxane,

1,3-bis-gamma-aminopropyltetraphenyl disiloxane,

and mixtures thereof.

The organic solvents utilized in accordance with this invention arethose organic solvents having functional groups which do not react witheither of the reactants, the aromatic diamines or the aromaticdianhydrides, to any appreciable extent. In addition to being inert withrespect to the reactants, the solvent utilized must be inert to and asolvent for the reaction product. In general, the organic solvent is anorganic liquid, other than either reactant or homologs of the reactants,which is a solvent for at least one of the reactants, and which containsfunctional groups other than monofunctional, primary and secondary aminogroups and other than the monofunctional dicarboxyl anhydro groups.Organic solvents having the foregoing characteristics with respect tothe reactants and reaction products are referred to herein as "inertsolvents." Such solvents include, for example, N-methyl-2-pyrrolidone(sometimes abbreviated NMP), dimethylsulfoxide (DMSO), N-formylmorpholine (NFM), or such organic solvents asN,N-dimethylmethoxy-acetamide, N-methyl-caprolactam, tetramethyleneurea, pyridine, dimethylsulfone, hexamethylphosphoramide,tetramethylenesulfone, formamide, N-methylformamide, N,N-dimethylformamide, butyrolactone, or N-acetyl-2-pyrrolidone. The solvents can beutilized alone, as mixtures, or in combination with relatively poorersolvents such as benzene, toluene, xylene, dioxane, cyclohexane, orbenzonitrile.

The volatile bases that are useful in connection with the presentinvention for producing a water soluble monomeric reaction product,include ammonia (NH₃), ammonium hydroxide (NH₄ OH), ammonium carbonate[(NH₄)₂ CO₃ ] and primary and secondary aliphatic amines containing upto four carbon atoms, such as methylamine, ethylamine, secondarybutylamine, isopropylamine, dimethylamine, diethylamine, dibutylamine,and the like.

In the initial reaction for preparing the coating composition embodyingthe present invention, an aromatic diamine is reacted with an aromaticdianhydride in the molar ratio of two-to-one respectively, or in otherwords in the ratio of two moles of the former to one mole of the latter.The reaction product may be expressed by the general formula: ##STR5##wherein the arrows denotes isomerism, that is where groups may exist ininterchanged positions, and R and R' are as defined above. Such amonomeric reaction product or a "polymer precursor" may be generallycharacterized as a "diamide-diacid-diamine". Upon additional of avolatile base, a compound having the following general formula results:##STR6## wherein X indicates the positive ion of the volatile base, andR and R' are as defined above. Such compound is water soluble so thatthe coating composition can be diluted with water to form anaqueous-organic coating medium.

To illustrate the present invention more specifically, the aromaticdiamine, 4,4'-diaminodiphenyl methane, also termed p,p-methylenedianiline (abbreviated "MDA" or simply "M"), was mixed with an aromaticdianhydride, 3,3',-4,4'-benzophenonetetracarboxylic dianhydride(abbreviated "BPDA" or simply "B"), in the molar ratio of two moles ofdiamine to one mole of dianhydride, in an anhydrousN-methyl-2-pyrrolidone (NMP) solvent at about 50% solids. The reactionwas spontaneous at a temperature below 70° C. The resulting product isthe monomer or "polymer precursor" having the formula ##STR7## whichformula may be conveniently abbreviated as "MBM". For more details onthe reaction of the diamine and dianhydride see U.S. Pat. Nos. 3,652,500and 3,663,510 referred to above.

Similarly, p,p'-methylene dianiline was condensed with4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate, in the molarratio of two-to-one, respectively in NMP solvent, at greater than 40%solids and at a temperature generally below 70° C. The resulting monomeror "polymer precursor" produced has the formula ##STR8## which monomericcompound may be abbreviated as "MAM".

Both the MBM and the MAM monomeric compounds are insoluble in water, butmay be made water soluble by the addition of a volatile base such asammonia or a volatile amine. The result is a water soluble diaminemonomer or polymer precursor. The water soluble material may be dilutedwith water to produce a coating medium having the desired solidscontent. For example, an aqueous NMP solution of the MAM or MBM monomerat 25% solids was applied to coat an aluminum substrate with a thin filmof the monomer. Quite unexpectedly, the films were found to be curableat temperatures between 150° C. and 250° C. to produce clear, non-tackyfilms with excellent adhesion to the substrate.

It should be noted that, in contrast to the effect of curing a thin filmcoating on a substrate, an attempt to further polymerize the monomer insolution by heating bulk solution in a reaction at a temperature in therange of 100° C.-120° C., results in precipitation of particles of themonomer, generally believed to be in the form of infusible, insolubleimides, namely intramolecular imidization products having the generalformula: ##STR9## wherein R and R' are as defined above.

Monomeric or "polymer precursor" compounds have been prepared fromvarious combinations of aromatic dianhydrides and aromatic diamines.Among such compounds are those prepared with the following molar ratio:2.0 moles 1,3-diamino benzene, also termed M-phenylene diamine, and 1.0mole 3,3',4,4' benzophenonetetracarboxylic dianhydride, 2.0 moles4,4'-diaminodiphenyl ether, also termed p,p'-oxydianiline, and 1.0 moleof 3,3',4,4'-benzophenonetetracarboxylic dianhydride; 2.0 molesm-phenylene diamine and 1.0 mole4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate; 2.0 molesp,p'-oxydianiline and 1.0 mole 4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydrotrimellitate. Such compounds were prepared in an N-methyl-2-pyrrolidone(NMP) solvent, ammonia or a suitable amine was added, and the solutionsdiluted with water to a 25% solids by weight solution. Thin films(0.2-0.5 mil.) were applied to a variety of substrates such as copper,aluminum, steel and the like, and the films were cured by heating toabove 150° C.

When properly cured, the coatings resulting in flexible, tough, clearfilms showing excellent adhesion to the aluminum substrate. These films,following thermal exposure such as 154 hours at 250° C., were found toexhibit low weight loss and excellent retention of adhesion andflexibility on aluminum. These properties were exhibited by films formedfrom the water insoluble versions of the coated compositions as well asfrom films formed from highly aqueous solvent systems. The various filmsand properties of films as applied to an aluminum substrate aresummarized in Table I.

                                      TABLE I                                     __________________________________________________________________________    Polymer Code                                                                           Components of                                                                          Thin film properties (0.2 to 0.5 mil on Al)                 aq.-NMP solv.                                                                          repeat mer of      15 min. at 150 C                                                                         Aging after cure*                      25% solids                                                                             polymer chain                                                                          15 in. at 150 C                                                                         + 30 min. at 250 C                                                                       154 hrs. at 250 C                      __________________________________________________________________________    MBM      2 MDA, 1 BPDA                                                                          clear-transparent                                                                       clear-transparent                                                                        clear-transparent                                        non-tacky non-tacky  with sl. darkening,                    MAM      2 MDA, 1 AGBA                                                                          excellent adh. to                                                                       excellent adh. to                                                                        excellent adhesion                                       Al substrate                                                                            Al substrate                                                                             to Al subtrate,                        OBO      2 ODA, 1 BPDA                                                                          flexibility un-                                                                         creasible  excellent flex. and                                      developed            very low wt. loss.                     OAO      2 ODA, 1 AGBA                 The wt. loss was                       OPO      2 ODA, 1 PMDA                 similar to polyimides                                                         and considerably less                                                         than occurs Alkanex                    mPBmP    2 mPDA, 1 BPDA                and Imidex**                           MPAmP    2 mPDA, 1 AGBA                                                       mPOmP    2 mPDA, 1 PMDA                                                       __________________________________________________________________________     where                                                                         M = p,p'-methylenedianiline                                                   O = p,p'-oxydianiline                                                         mP = M-phenylenediamine                                                       B =  3,3',4,4'-benzophenonetetracarboxylic dianhydride                        A = 4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate                     *The cure was arbitrarily selected as 15 min. at 150 C. followed by 30        min. at 250 C.                                                                **Alkanex is a General Electric Company trademark for its polyester           coating composition (U.S. Pat. No. 2,936,296) and Imidex is a General         Electric Company trademark for its polyesterimide coating composition.   

EXAMPLE 1

To a reactor equipped with a stirrer, nitrogen, atmosphere, entry port,and a thermometer well, was charged 132.2 g. N-methyl-2-pyrrolidonehaving a water content below 200 p.p.m. The NMP solvent was agitated and132.2 g. (0.667 moles) 4,4'-diaminodiphenyl methane (99% purity) wascharged over a period of about 30 sec. There resulted a clear solution"I". To a second similar reactor equipped with a heating mantle, wascharged 160.7 g. N-methyl-2-pyrrolidone having a water content below 200p.p.m. The NMP solvent was agitated and heated to a temperature of 60°C. whereupon with agitation 160.7 (0.333 mole)4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate (99% purity) wascharged over a period of about 3 min. There resulted a temperature riseto about 80° C. Stirring was continued for another 5 min. resulting in aclear, homogeneous solution "II". Solution II was cooled to about 43° C.and allowed to trickle into solution I over a period of about 2 min.with agitation. The temperature rose to a maximum of 75° C. during thenext 5 min. period of agitation. The precent imidization was found to be0.7 as determined by titration for the carboxylic acid content, inpyridine with tetrabutylammonium hydroxide and with thymol blue as theindicator. The resulting clear solution had a viscosity of 220 cps. anda solids level of 50.0% as the orthoamic acid. It was 47.9% solids asthe imide; the latter was determined by exposing 0.50 g. of samples inan aluminum cup having a diameter of about 2.5 inches to a temperatureof 150° C. for a period of 90 min.

EXAMPLE 2

To illustrate the effect of heating to imidization temperature, theorthoamic acid product of Example 1 was charged to a reactor equippedwith a stirrer, nitrogen atmosphere, thermometer well, heating mantle,and a condenser for collecting water of condensation. The product wasthen exposed to a temperature of 108° C. for a period of 2.5 hrs. duringwhich 11.2 g. water (0.62 mole) was collected. The material was titratedto carboxylic acid and the percent imidization found to be 93%. Oncooling, this solution showed considerable precipitation. Theprecipitate was collected and exposed to temperatures of 150° C. for 15min. followed by 250° C. for 30 min. but showed no signs of filmformation. Rather, it was observed that an infusible, sintered,sometimes powdered material was formed.

EXAMPLE 3

The preparation of Example 1 was repeated and the resulting 50.0%solution of the orthoamic acid, at a negligible level of imidization,was diluted with N-methyl-2-pyrrolidone to a solids level of 25%. Athin, wet film was applied to an aluminum, copper and iron substrate,the solvent was removed, and the film cured by heating for 90 min. at150° C. There resulted a clear tough film with excellent adhesion on thesubstrate. Additional heating at 250° C. for about 30 min. resulted in aflexible, clear, tough film with excellent adhesion. An 0.0403 inchaluminum round wire was coated, with the aid of appropriate sizes ofwire dies, in six passes with an intervening cure schedule per pass asdescribed above. The resulting film was at a total build of 3.0-3.2 milon the diameter, and exhibited a dielectric strength of 6000 v./mil.This coating further exhibited solvent resistance to a boiling 70/30mixture of denatured alcohol/toluol. A cut-through or thermoplastic flowtest was conducted on the coated wire. A 1000 g. weight was placed onthe crossover point of a pair of coated wires crossed at 90°. The samplewas positioned in a forced-air oven and the temperature allowed to rise5° C./min. until the wires made electrical contact under the 1000 g.load. The coating on aluminum had a cut-through temperature of 380° C.An infrared study showed strong spectral indications that the curedpolymer films are imides containing both amide and amine groups.

EXAMPLE 4

To 585.0 g. of the solution of Example 1 there was injectedsubsurfacewise and with agitation 75.0 g. of a 40% aqueous solution ofdimethylamine over a period of 2 min. The resulting solution was clearand dilutable with water. With agitation continuing, a mixture of 17.0g. ethylene glycol n-butyl ether, 5.8 g. N-methyl-2-pyrrolidone, 88.0 g.water, 35.2 g. n-butyl alcohol and sufficient nonylphenol ethylene oxideadduct to result ultimately in 45 p.p.m., was added resulting in a clearsolution having a solids level at 36.5 as the orthoamic acid in solutionand 36.0% as the imide in a cured film. The solution had a viscosity of185 cps., and a surface tension of 38.7 dynes/cm. The solution wasapplied to glass, aluminum and copper substrates with the aid of adoctor blade and exposed to a cure schedule of 15 min. at 150° C.followed by 30 min. at 200° C. and 30 min. at 250° C. The resulting 0.2to 0.5 mil films weree tough, clear, and exhibited excellent adhesion tothese substrates.

EXAMPLE 5

To 585 g. of the solution of Example 1 there was injected subsurfacewiseand with agitation 44.7 ml. of 28% ammonia water over a period of 2 min.The resulting solution was clear and dilutable with water. Withagitation continuing, a mixture of 17.0 g. ethylene glycol n-butylether, 5.8 g. N-methyl-2-pyrrolidone, 104.0 g. water, 35.2 g. n-butylalcohol and sufficient nonylphenol-ethylene oxide adduct to resultultimately in 45 p.p.m. was added, resulting in a clear solution havinga solids level of 37.6% as the orthoamic acid in solution and 36.0% asthe imide in a cured film. The solution had a viscosity of 224 cps., asurface tension of 39.5 dynes/cm., and a pH of 7.6 at 24° C. Thismaterial cured to a thin film 0.2 to 0.5 mil in thickness using a cureschedule of 150° C. for 15 followed by 250° C. for 30 min. and exhibitedexcellent adhesion on glass, copper, iron and aluminum.

The solution of Example 5 was die applied to 0.0403 inch aluminum roundconductor using appropriate wire die sizes, in six passes with anintervening cure per pass, to produce a total build of 2.83.2 mil on thediameter. A cut-through or thermoplastic flow test was conducted on thecoated conductor. A 1000 g. weight was placed on the cross-over point ofa pair of coated wires crossed at 90°. The set-up was positioned in aforced-air oven which allowed a temperature rise of 5° C./min. Thetemperature at which the pair of wires made electrical contact underthis load was found to be 362° C. When a sample of this coated wire wasplaced in a box of lead shot and exposed to an increasing voltage, theinsulating film was found to exhibit a dielectric strength of about 6000v./mil. The cured film was removed from another sample of the wire and athermogravimetric analysis was made with a duPont Model No. 900 Analyzerand the No. 950 TGA attachment. A 5 mg. sample was run at a heating rateof 5° C./min. in air. The results show that appreciable weight losscommences at about 340° C., at which point the loss of 6%. There is achange in slope at 400° C. where the weight loss was 19%. The 50% weightloss point was 555° C. This TGA behavior is somewhat comparable toelectrical grade polyesterimides. An infrared study showed strongspectral indications the cured polymer films were imides containingamide and amine groups.

EXAMPLE 6

To the first of the reactors of the type cited in Example 1 was charged132.2 g. N-methyl-2-pyrrolidone having a water content below 200 p.p.m.The NMP solvent was agitated and 132.2 g. (0.667 mole) of4,4'-diaminodiphenyl methane (99% was charged with agitation over aperiod of 30 sec. resulting in a clear solution. To the second reactorwas charged 429.4 g. N-methyl-2-pyrrolidone having a water content below200 p.p.m. The NMP solvent was was agitated and heated to a temperatureof 50° C. whereupon 107.3 g. (0.333 mole) of3,3',4,4'-benzophenonetetracarboxylic dianhydride was charged over aperiod of 2 min. with agitation. Stirring was continued for another 5min. and the solution allowed to cool to 30° C. The solution ofdianhydride was then trickled into the diamine solution with agitationover a period of 3 min. Stirring was continued for a period of 10 min.,resulting in a maximum temperature of 55° C. The material was titratedfor carboxylic acid and the percent imidization found to be less than1%. The resulting clear solution had a viscosity of 104 cps. at 24° C.at a solids level of 29.9% as the orthoamic acid solution and 28.4% asthe imide (cured) film, the latter being determined by exposing one gramof sample in an aluminum cup having a diameter of about 2.5 inches to atemperature of 150° C. for a period of 90 min. When a film of thesolution was formed on aluminum, iron, copper or glass and exposed totemperatures of 150° C. for 15 min. followed by 250° C. for 30 min.,there resulted a smooth, clear, tough adhering film.

EXAMPLE 7

To a reactor equipped with a thermometer well, heating mantle and acondenser for collecting water of condensation from the reactor wascharged the entire set of raw materials in sequence as per Example 6.The resulting 29.9% orthoamic acid solution was then exposed to atemperature of 110° C. for a period of 2.5 hrs. during which 11.7 g.(0.65 mole) water was collected. On cooling this solution to 25° C.there appeared considerable flocculated-insoluble material. Thismaterial was titrated for carboxylic acid and the percent imidizationwas found to be about 98%. When this material was exposed to 150° C. for15 min. followed by 250° C. for 30 min. there resuled sinteredbead-to-powder like, infusible material. The material was not a filmformer and showed little or no signs of adhesion to any substrate. Theinfrared spectrum indicated imide with absence of amide groups.

EXAMPLE 8

To 200.0 g. portions of a large batch of the solution prepared asdescribed in Example 6 was added a mixture of 9.5 g. n-butyl alcohol,0.5 g. N-methyl-2-pyrrolidone and sufficient nonylphenol-ethylene oxideadduct to provide a level of 90 p.p.m. of this material in the system.The resulting clear solution had a viscosity of 80 cps. at 24° C. at asolids level of 28.5% as the orthoamic acid and 27.0% as the imide. An0.0403 inch diameter copper conductor and an 0.0403 inch diameteraluminum conductor wire was coated in a conventional wire enamellingtower at a wire speed of about 40 ft./min. using appropriate wire dies,such that the overall build was 2.8-3.2 mil on the diameter. Anelectrical burnout test was conducted on the resulting coated copperconductor using a Techrand Wire Burnout Tester Model HBT-1. The test wasconducted on a twisted pair of wires having nine twists per five inchesmade under three pounds tension. The amperage was set at 36 amps.initially and increased two amps every 180 sec. The sample ran for 1231sec. before failure.

In another electrical property evaluation of the film coated on thecoated 0.0403 copper conductor, and also a similarly coated 0.0403aluminum conductor, a cut-through or thermoplastic flow test was made. A1000 g. weight was placed on the crossover point of a pair of coatedwires crossed at 90°. The setup was positioned in a forced-air ovenequipped with necessary thermocouples and recorders, and the temperatureallowed to rise at a rate of 5° C./min. In this test an electricalcircuit is completed between the two crossed wires when the temperatureis reached such that the wires cut-through under the 1000 g. weight,that temperature is noted as the cut-through temperature. The coated0.0403 inch copper and aluminum conductors were exposed to temperatureincreases to 500° C., the limit of the apparatus, and showed no sign offailure. By way of comparison a typical electrical grade polyesterinsulation for this wire size with this weight would generally fail inthe temperature range of 220° to 270° C., polyesterimides wouldgenerally fail in the range of 340° to 400° C., and polyamides would notfail at 500° C.

Other properties of the coated wire included 0-5 breaks per 100 ft. ofwire at 3000 v. using a standard continuity tester. Using a boxcontaining lead shot a wire was exposed to increasing voltages at therate of 500 v./sec. and found to withstand 9000 v. before failure of6000 v./mil. Solvent resistance was evaluated using a 70/30 mixture ofdenatured alcohol and toluol by immersing wire sample for five min. atthe boiling point with no evidence of swelling or blistering. Aninfrared spectrum of the insulating film showed strong spectralindications that the cured polymer film is an imide containing amide andamine groups, or possibly a polyamideimide containing amine groups.

EXAMPLE 9

To 400 g. portions of a large batch of solution prepared as described inExample 6 was added 22.4 ml. of 28% aqueous ammoniacal solutionsubsurface wise and with agitation, over a period of 1.5 min. To thereactor was then charged, for each 400 g. portion 21.1 g. of a mixtureof 95% n-butyl alcohol and 5% N-methyl-2-pyrrolidone and sufficientamount of nonylphenol-ethylene oxide adduct such that the resultingtotal system had about 60 p.p.m. of the latter component. There resulteda clear solution having a viscosity of 112 cps. at 24° C. at a 27.0%solids level as the orthoamic acid and 25.6% as the imide. The latterwas determined by exposing a thin film of the liquid to 150° C. for aperiod of 90 min. This solution was water reducible. Round copper andaluminum conductors were coated in a conventional wire enamelling towerat a wire speed of about 40 ft./min. using seven appropriate wire dies,resulting in a build on the diameter of 2.8-3.2 mil. Using the burnouttest described in Example 8, the twisted pair sample of the copperconductor withstood 1260 sec. before failure. Using the test procedurefor determining cut-through temperature as cited in Example 8, thecoated copper and aluminum conductors were found to have a cut-throughtemperature in excess of 500° C. in both cases. Other properties of thiswire included 0-4 breaks per 100 ft. of wire at 3000 v. using thestandard continuity tester, and a dielectric strength of about 6000v./mil.

The cured film was removed from another sample of the wire and athermogravimetric analysis made with a duPont Model No. 900 Analyzer andthe No. 950 TGA attachment. A 5 mg. sample was run at a heating rate of5° C./min. in air. The results showed that appreciable weight losscommenced at about 500° C. where the weight loss was 6%. The 50% weightloss point occurred at 570° C. This is comparable to conventional highmolecular weight aromatic polyimides made from1,2,4,5-benzenetetracarboxylic dianhydride and 4,4'-diaminodiphenylether, and from 3,3',4,4'-benzophenonetetracarboxylic dianhydride and4,4'-diaminodiphenyl methane. An infrared spectrum of the film showedstrong sprectral indications that the polymer is an imide containingamide and amine groups or possibly a polyamideimide containing aminegroups.

EXAMPLE 10

To a reactor equipped with a stirrer, nitrogen atmosphere, entry port,and a thermometer well was charged 108.1 g. N-formyl morpholine having awater content below 220 p.p.m. The solution was agitated and 108.1 g.(1.0 mole) m-phenylene diamine was charged over a period of about 30sec. There resulted a clear solution I. To a second similar reactorequipped with a heating mantle, was charged 161.0 g. N-formyl morpholinehaving a water content below 200 p.p.m. The solution was agitated andheated to a temperature of 50° C. whereupon with agitation 161.0 g.(0.50 mole) 3,3',4,4'-benzophenonetetracarboxylic dianhydride wascharged over a period of 3 min. There resulted a temperature rise to 76°C. Stirring was continued for another 5 min. resulting in a clearhomogeneous solution II. The solution II was cooled to about 40° C. andallowed to trickle into solution I over a period of about 2 min. withagitation. The temperature rose to a maximum of 73° C. during the next 5min. period of agitation. The resulting clear solution had a viscosityof 290 cps. and a solids level of 50.0% as the orthoamic acid.

EXAMPLE 11

A portion of the solution of Example 10 was diluted with N-formylmorpholine to a solids level of 23.5%. Wet films were metered on glassand on aluminum substrates with the aid of a doctor blade such thattreatment with a cure schedule of 30 min. at 150° C., 30 min. at 200° C.and 30 min. at 250° C. resulted in a film build of 0.2 to 0.5 mil. Thefilms were clear, tough, very flexible on the bendable substrate, andexhibited excellent adhesion to both substrates.

EXAMPLE 12

To 269.1 g. of the 50.0% solids solution of Example 10 was injectedsubsurfacewise and with agitation a mixture of 100 ml. water and 33.7ml. of 28% ammonia water over a period of 2 min. The resulting solutionwas clear and dilutable with water. While continuing agitation, amixture of 23.4 g. n-butyl alcohol, 2.2 g. N-formyl morpholine, 100 ml.water and sufficient nonylphenol ethylene oxide adduct to provide 60p.p.m. in the total formulation was added, resulting in a clear solutionhaving a solids level at 25.9% as the orthoamic acid and 24.2% as theimide. The solution had a viscosity of 288 cps., surface tension of 37.0dynes/cm., and a pH of 7.4 at 24° C. When cured as a 0.2 to 0.5 mil filmon aluminum, copper, or iron substrates, using a cure schedule of 150°C. for 15 min., 300° C. for 30 min. and 250° C. for 30 min., thereresulted a clear, tough and flexible film.

EXAMPLE 13

To a reactor equipped with a stirrer, nitrogen atmosphere, entry portand a thermometer well was charged 200.4 g. N-methyl-2-pyrrolidone. TheNMP solvent was agitated and 200.4 g. (1.0 mole) 4,4'-diaminodiphenylether was charged over a period of 30 sec. There resulted a clearsolution I. To a second similar reactor equipped with a heating mantlewas charged 161.0 g. N,N-dimethylformamide (DMF). The DMF solvent washeated to a temperature of 50° C. whereupon with agitation 161.0 g.(0.50 mole) 3,3',4,4'-benzophenonetetracarboxylic dianhydride wascharged over a period of about 3 min. There resulted a temperature riseto about 70° C. Stirring was continued for another 5 min. resulting in aclear homogeneous solution. The solution was cooled to about 37° C. andallowed to trickle into solution I over a period of about 3 min. withagitation. The temperature rose to a maximum of 68° C. during the next10 min. period of agitation. The resulting clear solution was at 214cps. and a solids level of 50.0% as the orthoamic acid. The percentimidization was determined from a titration of the carboxylic acidgroups and found to be 1.2%.

EXAMPLE 14

A portion of the solution of Example 13 was diluted withN-methyl-2-pyrrolidone to a solids level of 19.0% and a wet film drawnon aluminum strip with the aid of a doctor blade. Upon application of acure schedule of 30 min. at 150° C., 30 min. at 200° C. and 30 min. at250° C. there resulted a clear, tough, flexible film exhibitingexcellent adhesion.

EXAMPLE 15

To a reactor equipped with a stirrer, nitrogen atmosphere, entry portand a thermometer well was charged 108.1 g. N-methyl-2-pyrrolidonefollowed by 108.1 g. (1.0 mole) m-phenylene diamine, resulting in aclear solution I. To a second similar reactor equipped with a heatingmantle was charged 241.0 g. N-methyl-2-pyrrolidone. The NMP solvent washeated to 55° C. whereupon with agitation 241.0 g. (0.50 mole)4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate was charged over aperiod of 3 min. and stirring continued for an additional 10 min. periodduring which the temperature reached a maximum of 68° C. The solutionwas cooled to about 30° C. and allowed to trickle into solution I over aperiod of about 3 minutes with agitation. The temperature peaked at 65°C. during the additional 15 minute period of agitation. The resultinghomogeneous solution in the reactor was allowed to cool to 35° C.whereupon with agitation a mixture of 200 ml. water and 67.0 ml. of 28%ammonia water was injected subsurfacewise over a period of 2.5 min. Withagitation continuing a mixture of 50.0 g. n-butyl alcohol, 4.0 g.N-methyl-2-pyrrolidone, 200 ml. water sufficient nonylphenolethyleneoxide adduct to provide 45 p.p.m. was added. The resulting clearsolution hd a solids level of 29.0% as the orthoamic acid in solutionand 27.4% as the imide in a cured film. The solution had a viscosity of175 cps., surface tension of 36.8 dynes/cm., and a pH of 7.1 at 24.5° C.A 0.2 to 0.5 mil film formed on aluminum strip with a cure schedule of15 min. at 150° C., 30 min. at 200° C. and 30 min. at 250° C. was foundto be clear, adherent, tough and flexible.

EXAMPLE 16

To the first of the reactors of the type referred to in Example 10 wascharged 132.2 g. N-formyl morpholine (NFM) having a water content below200 p.p.m. The NMF solvent was agitated and 132.2 g (0.667 mole) of4,4'-diaminodiphenyl methane (99% purity) was charged resulting in aclear solution I. To a second similar reactor equipped with a heatingmantle was charged 429.4 g. N-formyl morpholine having a water contentbelow 200 p.p.m. The NMF solvent was agitated and heated to atemperature of 58° C. whereupon 107.3 g. (0.333 mole) of3,3',4,4'-benzophenonetetracarboxylic dianhydride was charged over aperiod of 4 min. with agitation and the stirring continued fr anadditional period of 15 min. After cooling to 28° C. the solution ofdianhydride was trickled into the diamine solution in the first reactorwith agitation over a period of 7 min. The stirring was continued for aperiod of about 15 min. The maximum temperature was 74° C. The resultingclear solution was titrated for carboxylic acid and the percentimidization found to be 0.6%. The contents of the reactor was allowed tocool to 32° C. To the reactor was added 65.6 g. of a 60% aqueoussolution of isopropylamine, subsurfacewise and with agitation over aperiod of 2.5 min. To the reactor was then charged 42.0 g. of a mixtureof 95% n-butyl alcohol and 5% N-formyl morpholine and sufficientnonylphenol-ethylene oxide adduct such that the resulting formulationwas at 50 p.p.m. with respect to the nonionic surfactant. There resulteda clear solution having a viscosity of 278 cps. at 25° C. at a solidslevel of 27.5% as the orthoamic acid. The solution was water reducible.A 0.2 to 0.5 mil film formed on aluminum strip with a cure schedule of15 min. at 150° C., 30 min. at 220° C. and 30 min. at 250° C. was clearand tough, and exhibited excellent adhesion and flexibility.

REACTION ROUTE

Upon curing the coatings of the diamide-diaciddiamine coating mediumabove described, after application thereof as thin films to substrates,such as, for example, the application of the medium to form a thin filmcoating on wire, at least two possible reaction routes, and moreprobably a mixture of both, may account for the properties that havebeen observed for the coated films. To account for the unexpectedflexible films produced as well as for the loss of two moles of waterper monomeric unit upon cure of a coating thereof (see Examples 2 and6), if it is assumed that intermolecular imidization occurs upon heatingthe diacid-diamide-diamine thin film coating, in contrast to theintramolecular imidization which occurs upon heating the monomer in bulkor in solution, and also assuming for purposes of illustration the useof the MBM monomeric coating medium, it is possible that, upon heatcure, a ladder polymer structure results having benzophenone rungs andaromatic polyimide side posts with pendant amines, which structure maybe written as follows: ##STR10##

Such a structure, based on intermolecular imidization, could account forthe flexibility and other properties unexpectedly obtained upon curingcoatings and films of the monomer, in contrast to the non-flexible,infusible, sintered-powder particles obtained through intramolecularimidization. That an imidization reaction occurs is noted in the weightloss of exactly two moles of water per mole of MBM. It should also benoted that two moles of water are also lost in the intramolecularimidization reaction which results in the formation of an infusiblepowder (see Examples 2 and 6).

On the other hand, if any intermolecular amidization results, againassuming a thin film coating of MBM monomeric coating medium, it ispossible that, upon heat cure, a ladder polymer results havingbenzophenone rungs and aromatic polyamide side posts, which polymerstructure may be written as follows: ##STR11##

More probably, a mixture of the ladder polyimidepolyamine structure andthe ladder polyamide structure results upon heat cure of the coating orfilm of the diamidediacid-diamine monomer. That this is the case issupported by preliminary infrared analysis results which reveals thepresence, simultaneously, of amine groups, amide groups, and imidegroups, and do not show the presence of free carboxyl groups. Althoughthe above structures have been written for the MBM monomer, similarpolymer structures analogous to those written above can be readilywritten for each of the monomeric compounds referred to above. In allcases, it is believed that a ladder type polymeric structure results.

While certain illustrative compositions and methods embodying thepresent invention have been described above in considerable detail, itshould be understood that there is no intention to limit the inventionto the specific embodiments which have been disclosed by way ofillustration and not by way of limitation. On the contrary, theintention is to cover all modifications, alternatives, equivalents anduses falling within the spirit and scope of the invention as expressedin the appended claims.

I claim as my invention:
 1. A heat curable coating medium comprising anaromatic diamide diacid diamine formed as the reaction product of anaromatic diamine and an aromatic dianhydride, in the molar ratio oftwo-to-one respectively, at a temperature below that at which more thannegligible imidization occurs, in an inert solvent for at least one ofsaid reactants, a coating of said medium on a substrate being heatcurable at imidization temperature to form a cured highly cross-linkedpolymeric coating thereon.
 2. A coating medium comprising an aromaticdiamide diacid diamine formed as the reaction product of an aromaticdiamine and an aromatic dianhydride, in the molar ratio of two-to-onerespectively, at a temperature below that at which more than negligibleimidization occurs, a volatile base, and an aqueous-organic solventmedium wherein the organic portion of said solvent medium is a watermiscible inert solvent for at least one of said reactants, said reactionproduct being heat curable at imidization temperature after applicationof said coating medium as a thin film coating on a substrate, to form ahighly cross-linked polymeric coating including imide groups, amidegroups and amine groups and being substantially free of carboxyl groups.3. The coating medium defined in claim 2 wherein:said dianhydride isselected from the group consisting of:4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, bis(2,3-dicarboxyphenyl) methane dianhydride,2,2-bis (3,4-dicarboxyphenyl)propane dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride,2,2-bis(2,3-dicarboxyphenylpropane dianhydride,1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, and the like. and saiddiamine is selected from the group consisting of: meta-phenylenediamine, para-phenylenediamine, 4,4'-diamino-diphenyl propane,4,4'-diamino-diphenyl methane, benzidine, 4,4'-diamino-diphenyl sulfide,4,4'-diamino-diphenyl sulfone, 3,3'-diamino-diphenyl sulfone,4,4'-diamino-diphenyl ether, 2,6-diamino-pyridine,bis-(4-amino-phenyl)diethyl silane, bis-(4-amino-phenyl) phosphineoxide, bis-(4-amino-phenyl)-N-methylamine, 1,5-diamino naphthalene,3,3'-dimethyl-4,4'-diamino-biphenyl, 3,3'-dimethoxy benzidine, m-xylenediamine, p-xylene diamine, 1,3-bis-delta-amino-butyltetramethyldisiloxane, and 1,3-bis-gamma-aminopropyltetraphenyl disiloxane.
 4. Thecoating medium defined in claim 2 wherein:said diamine is selected fromthe group consisting of 4,4'-diaminodiphenyl methane,4,4'-diaminodiphenyl ether, and 1,3-diamino benzene, and saiddianhydride is selected from the group consisting of3,3',4,4'-benzophenonetetracarboxylic dianhydride and4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate.
 5. A heat curablecoating medium comprising an aromatic diamide diacid diamine formed asthe reaction product of an aromatic diamine and a tetracarboxyliccompound, in the molar ratio of two-to-one respectively, at atemperature below that at which more than negligible imidization occurs,in a inert solvent for at least one of said reactants, a coating of saidmedium on a substrate being heat curable at imidizaton temperature toform a cured highly cross-linked polymeric coating thereon.
 6. A coatingmedium comprising an aromatic diamide diacid diamine formed as thereaction product of an aromatic diamine and a tetracarboxylic compound,in the molar ratio of two-to-one respectively, at a temperature belowthat at which more than negligible imidization occurs, a volatile base,and an aqueous-organic solvent medium wherein the organic portion ofsaid solvent medium is a water miscible inert solvent for at least oneof said reactants, said reaction product being heat curable atimidization temperature after application of said coating medium as athin film coating on a substrate, to form a highly cross-linkedpolymeric coating including imide groups, amide groups and amine groupsand being substantially free of carboxyl groups.
 7. The coating mediumdefined in claim 6 wherein: said tetracarboxylic compound is adianhydride selected from the group consistingof:4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate,3,3'-4,4'-benzophenonetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl) sulfone dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, 2,2-bis (3,4-dicarboxyphenyl)propane dianhydride,bis (3,4-dicarboxyphenyl) ether dianhydride, 2.2-bis(2,3-carboxyphenyl)propanedianhydride,1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,1,1-bis(3,4-dicarboxylphenyl)ethane dianhydride, and the like, and saiddiamine is selected from the group consisting of: meta-phenylenediamine, para-phenylenediamine, 4,4'-diamino-diphenyl propane,4,4'-diamino-diphenyl methane, benzidine, 4,4'diamino-diphenyl sulfide,4,4'-diamino-diphenyl sulfone, 3,3'-diamino-diphenyl sulfone,4,4'-diamino-diphenyl ether, 2,6-diamino-pyridine, bis-(4-amino-phenyl)diethyl silane, bis-(4-amino-phenyl) phosphine oxide,bis-(4-amino-phenyl-N-methylamine, 1,5-diamino naphthalene,3,3'-dimethyl-4,4'-diamino-biphenyl, 3,3'-dimethoxy benzidine,m-xylylene diamine, p-xylyene diamine,1,3-bis-delta-amino-butyltetramethyl disiloxane, and1,3-bis-gamma-aminopropyltetraphenyl disiloxane.
 8. The coating mediumdefined in claim 7 wherein: said diamine is selected from the groupconsisting of 4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl ether,and 1,3-diamino benzene, andsaid dianhydride is selected from the groupconsisting of 3,3',4,4'-benzophenonetetracarboxylic dianhydride and4,4'-(2-acetoxy-1,3-glyceryl) bis-anhydro trimellitate.
 9. A heatcurable coating material as defined in claim 1 wherein said aromaticdiamine is selected from the group consisting of diamine having thegeneral formula:

    H.sub.2 N--R'-NH.sub.2

wherein R' is a divalent radical selected from the group consisting of##STR12## wherein R'" and R"" are an alkyl or an aryl group having 1 to6 carbon atoms, n is an integer of from 1 to 4 and m has a value of 0, 1or more and ##STR13## wherein R" is selected from the group consistingof an alkylene chain having 1-3 carbon atoms, ##STR14## wherein R'" andR"" are as above-defined and x is an integer of at least 0; and saidaromatic dianhydride is selected from the group consisting ofdianhydrides having the general formula: ##STR15## where R is atetravalent radical containing two benzene rings joined by anon-aromatic non-cyclic group, each pair of carboxyl groups beingattached to different adjacent carbon atoms of a single separate ring.10. A heat curable coating material as defined in claim 1 wherein saidaromatic diamine is selected from the group consisting of4,4'-diaminodiphenyl ether, and 1,3-diamino benzene and mixturesthereof, and said aromatic dianhydride is selected from the groupconsisting of 3,3', 4,4'-benzophenonetetracarboxylic dianhydride and4,4'-(2-acetoxy-1,3-glyceryl)bis-anhydro trimellitate and mixturesthereof.
 11. The heat curable coating medium as defined in claim 1wherein said aromatic diamide diacid diamine reaction product is##STR16##
 12. The heat curable coating medium as defined in claim 1wherein said aromatic diamide diacid diamine reaction product is##STR17##